Biosensors are used for a large number of different research, industrial, and medical applications. Such sensors detect analyte molecules of interest, in particular biological molecules and macromolecules, by a variety of different mechanisms. Some biosensors react enzymatically with analyte molecules, while other biosensors capture analyte molecules using different types of antibody or nucleic acid capture agents.
One thing that all biosensors have in common is that after the biosensor detects an analyte molecule, the biosensor then typically produces a detectable signal, such as a fluorescent, colorimetric, or electronic signal, which in turn can usually be detected by suitable photometric or electrical signal sensing instrumentation.
Biosensors are typically either single use or multiple use. Although a multiple use biosensor is clearly preferable to a single use biosensor, it isn't always technically feasible to reuse a biosensor. Some biosensors, particularly enzymatic biosensors, are inherently single use because the biosensor's enzyme or enzyme substrate component becomes exhausted during the first sensing run. Even for non-enzymatic biosensors, such as antibody or nucleic acid based biosensors, the biosensor may become fouled by the target analyte molecules, preventing the sensor from being reused.
One interesting class of biosensors is based upon field-effect transistors (FET). The conductivity of the FET transistor can be greatly affected by very small changes in the electrical properties of the FET's “gate” terminal component. Thus by binding suitable detection components to the FET gate, a FET based biosensor can be created in which the binding of analyte biological molecules to detection group molecules (which are bound to the FET gate), modulate the conductivity of the FET transistor. Due to the high electrical amplification characteristics of a FET, the binding of a relatively few biological target analyte molecules to the FET gate bound detection group molecules creates a substantial change in the electrical conduction characteristics of the FET. This change can then be detected and reported by suitable electronic instrumentation.
Although a number of different materials can be used to create FETs, silicon is one of the most common materials used for this purpose.
As efforts to increase the sensitivity of FET transistors have progressed, the geometry of the FET transistor has changed. It was found that as the geometry of the FET transistor became smaller and more wire-like, a number of favorable effects occurred. The FET gate, where the biological molecule detecting groups or elements are bound, is typically on the outside of the FET. Binding of such molecules to the gate affects the conductivity of the FET body material on the inside of the FET. As the size of the FET shrinks, the ratio of the gate (surface) to body (volume) grows, and thus molecules bound to the gate exert a greater effect on the electrical conductivity of the FET body material. As the length of the FET grows, there is more opportunity for molecules bound to the gate to alter the conductivity of the FET body. The longer length also provides more room to bind a larger number of detection group or element molecules to the FET gate, increasing the sensitivity of the FET still more.
As a result, recent work in the field has increasingly focused on biosensing FETs and other devices where the geometry of the FET has been shrunk and stretched to create near atomic scale wires with lengths on the order of 4-50 microns (1 μm=1000 nanometers) and widths and heights on the order of 50-100 nanometers. Such devices are called nanowires, or (if silicon is used as the device substrate) silicon nanowires.
Some recent reviews of nanowire biosensors include Patolsky et. al., “Nanowire sensors for medicine and the life sciences”, Nanomedicine (2006), 1(1), 51-65; and Li et. al., “Silicon nanowires for sequence-specific DNA sensing: device fabrication and simulation,” Appl. Phys A 80, 1257-1263 (2005).
Although, on a theoretical level, silicon nanowires appear to be a promising biosensor approach, there are still a number of issues that must be resolved in order to make such technology practical for routine use.
One of these practical problems is the problem of creating long lifetime, reusable, nanowire biosensors. When the biosensors are freshly made, the detecting element molecules (such as antibodies or nucleic acids) that bound to the surface of the nanowire are initially unoccupied by their target analyte molecules, and thus a fresh biosensor has very high sensitivity. However after the first sensing session, a number of the analyte (target) molecules may remain bound to the detecting elements, rendering these elements inactive. As the number of sensing sessions increases, the number of free (no previously bound analyte) detecting elements decreases, and eventually the biosensor becomes too insensitive to be useful (fouled).
In order to restore nanowire sensitivity, various schemes have been proposed. These schemes include regenerating the nanowire biosensor by flushing it with excess amounts of fresh buffer that free from analyte (target) molecules. Although such methods are likely to be effective at removing very weakly bound target analyte molecules, they are likely to be ineffective at removing more strongly bound target analyte molecules, such as nucleic acid analyte molecules.
Other methods to regenerate nanowire biosensors, such as flushing the biosensor with a binding disruption buffer (typically a low salt buffer, a high salt buffer, a detergent, a chaotropic agent, or other agent) can be cumbersome and problematic for many applications. Thus improved methods to more easily regenerate nanowire biosensors are desirable.